British Prime Minister David Cameron was forced to abandon a visit to a military base in Afghanistan on Thursday after intelligence suggested he could be targeted in an insurgent rocket attack.He was due to visit troops on a forward operating base in Helmand province, but the trip was abruptly cancelled after intelligence intercepted threatening telephone exchanges by insurgents.Making his first visit to Afghanistan as prime minister, Cameron earlier ruled out increasing Britain's troop commitment and called for quicker progress to bring troops home. The British government is facing increasing public opposition to the long and costly war in Afghanistan, where 294 British personnel have died since operations began to topple the Taliban government in October 2001.